Nisus \Ni"sus\, n. [L., fr. niti, p. p. nisus, to strive.] A striving; an effort; a conatus.
A nisus or energizing towards a presented object. --Hickok.
Source: Webster's Revised Unabridged Dictionary (1913)
Nisus \Ni"sus\, n. (Physiol.) (a) The periodic procreative desire manifested in the spring by birds, etc. (b) The contraction of the diaphragm and abdominal muscles to evacuate feces or urine.